The present invention relates to comminuting and classifying apparatus. More particularly, it relates to a comminuting and classifying mill of the re-entrant circulating stream jet type commonly referred to as "Micronizers". The Micronizer is the oldest and most widely used of the re-entrant circulating stream jet grinding mills.
This type of grinding mill is described in detail in U.S. Pat. No. 2,032,827 issued Mar. 3, 1936. The basic mill includes a vortex chamber comprising an annular peripheral wall closed by two opposed lateral walls. In its preferred form, the vortex chamber is formed so that the axial length of the peripheral wall is only a small fraction of the diameter of the chamber. The peripheral wall is surrounded by a manifold through which high pressure gas is supplied to a plurality of gaseous fluid nozzles positioned around the peripheral wall and angled so that the gaseous stream issuing from them propels the fluid with both a forward and a transverse component of movement relative to the axis of the chamber. Conventionally, the material to be comminuted is fed into the chamber by jet and venturi apparatus located near the periphery of the chamber.
The introduction of feed material into the Micronizer has been a continuing problem since it was first put into commercial use. These problems are discussed in some detail in U.S. Pat. No. 4,018,388 issued Apr. 19, 1977. The principal problems involve the distribution of the feed material in the proper zone and its effect upon the whirling vortex of fluid within the chamber.
Jet feeding means are mainly concerned with providing a more uniform distribution of feeding the material into the grinding zone although they do have the advantage of giving the feed material and its carrier fluid an initial velocity. They are principally preferred for comminuting various types of free-flowing materials because damp, viscous, or precipitate materials frequently tend to clog the feed apparatus.
The problem with screw-feeding material into the chamber arises from the fact that the circulating gases within the chamber have little or no momentum. The introduction of feed material in a static condition results in a localized shock to the conformity of flow of the circulating gases as they try to accelerate the feed material from the static condition to the velocity of the circulating load in the mill.
The apparatus described in U.S. Pat. No. 4,018,388 has been a commercially successful solution to the problem of improving the distribution of the feed material as it is introduced into the re-entrant circulating stream jet grinding mill. As described in the patent, a conical recess is provided in the side wall opposite the outlet and the feed material is axially introduced into the recess so that it disperses from the apex of the cone radially and axially toward the classification zone of the circulating gases. The feed material becomes entrained with the circulating gases, the vortex of which extends into the conical recess. Thus, the feed material mixes with the circulating gases and there is a uniform distribution of the feed material into the classification zone.
Although the mill described in U.S. Pat. No. 4,018,388 is indeed a successful improvement in feed distribution in this type of mill, the fact remains that the fluid gas and entrained feed material is directed radially and axially in a random manner with no circulating direction. This means that the energy necessary to give it a high velocity whirling effect must be extracted from the classifying vortex. Indeed, the patent expressly points out that it uses the circular velocity of the gases in the classification zone to accelerate the feed material to a velocity in excess of the circulating load adjacent the inner periphery of the mill. See U.S. Pat. No. 4,018,388, column 3, lines 33-48.
The present invention is therefore concerned with providing all of the benefits and advantages of the re-entrant circulating stream jet grinding mill described in U.S. Pat. No. 4,018,388 without extracting energy from the classifying vortex circulating within the chamber. Thus, it is one of the objects of the present invention to feed material with its carrier gas so as to provide it with a rotative velocity which will be transmitted to the classifying vortex thereby supplementing its velocity rather than extracting energy from it.